1. Field of Invention
The present invention relates to a method and device for monitoring a patient's breathing during magnetic resonance imaging (MRI).
2. Description of Prior Art
Patients undergoing an MRI exam are generally left isolated in a shielded room during the imaging procedure. When a patient is placed in the bore of the MRI magnet, he or she is nearly completely screened from view. The patient's movements, which are commonly used as indicators of the patient's well-being, are extremely difficult to observe. This creates concerns when imaging patients at risk of untoward complications while within the bore of the magnet.
There are three classes of patients which warrant careful attention during MRI imaging. First, a large percentage of patients develop mild to severe claustrophobia while within the confines of the magnet bore. It is common practice to assess their status by speaking with them over an intercom system. More sophisticated types of monitoring are not considered cost effective for these patients. Second, more serious concerns arise when gadolinium contrast agents are administered, when agents for conscious sedation (such as benzodiazapines or synthetic opiates) are administered, or when patients are heavily sedated. Monitoring of these patients consists generally of a nurse standing by, who intermittently manually checks vital signs. Occasionally, an MRI compatible oxygen saturation monitor is placed on these patients, giving a delayed and nonspecific measure of respiratory status. Third, patients that are medically unstable often require continuous monitoring of primary physiological parameters or they cannot be imaged at all.
In general, a patient's breathing is used as a primary means of assessment. Changes in breathing usually occur in advance of more serious complications evidenced by decreased oxygen saturation, respiratory acidosis, mental agitation and cardiac disturbances including cardiac arrest. Respiration monitoring, however, often appears to be given a secondary role to more sophisticated measurements such as oxygen saturation, blood pressure, or heart rate. This is a holdover from other medical disciplines where the patient is readily accessible and the presence of breathing is usually quite obvious. Students of cardiopulmonary resuscitation (CPR) know that the oxygen saturation, blood pressure, etc. are quite irrelevant when presented with a patient who is not breathing.
Conventional patient monitoring equipment is unsuitable for use in or near an MRI system because: 1) it puts patients at risk for burns due to eddy currents generated within conductive parts, 2) conventional monitoring equipment frequently gives erroneous indications or malfunctions in the presence of the large static and changing magnetic fields, and radio frequency (RF) electric fields generated by an MRI system, and 3) conductive or magnetic parts can cause unacceptable distortion of the diagnostic images.
Within the context of the present invention, the words conductive and magnetic have the following meaning: A part is considered "conductive" if its electrical conductivity is sufficiently high such that the currents induced in said part by the magnetic fields present in an MRI system cause either significant heating within said part or detectable distortion of images of said MRI system. A part is considered "magnetic" if its magnetic susceptibility is sufficiently high or its magnetic permeability is sufficiently different from a value of 1.0 such that detectable distortion of images of said MRI system occurs.
At present, there do exist monitors which have adapted conventional monitoring technology to the MRI environment. For example, U.S. Pat. No. 5,323,776 to Blakely et al (1994) describes an MRI compatible pulse oximetry system. In addition, Invivo Inc. presently sells an MRI compatible system which measures several patient parameters. Both systems, however, make use of conductive parts, making the placement of sensors and associated cabling problematic. In addition, these systems are expensive and complex, and as such are not commonly used outside of the most sophisticated imaging centers and hospitals. At present, many patients who could benefit from MRI imaging are either put at risk for lack of any monitoring, or are ineligible for the procedure for lack of cost effective MRI compatible monitoring equipment.
For certain MRI imaging protocols, there is a need to synchronize the imaging process with the patient's breathing. Motion artifacts in MRI images due to movement of the patient can be reduced by collecting image data at the same point in the patient's respiratory cycle. This is generally accomplished by monitoring chest wall movement with a mechanical transducer wrapped around the patient's chest. Although these chest wall movement sensors are considered adequate for the reduction of motion artifact, they are not suitable for monitoring the patient's exchange of respiratory air. This is because chest wall movement does not necessarily correlate with the exchange of air in a patients lungs. For example, shallow breathers, diaphragm breathers, and emphysematous patients often exhibit small or anomalous chest wall movements as air in their lungs is exchanged. In addition, patient movement unrelated the breathing will falsely trigger these type sensors. Chest wall movement sensors are, therefore, not a reliable monitor of a patients well being. Conversely, a respiration sensor which does sense the exchange of respiratory air may provide a useful signal for respiratory gating. It will be more sensitive to subtle movements of the lungs and insensitive to patient movement unrelated to breathing.
In the absence of large magnetic or RF electric fields, many methods of sensing respiration have been used. One particular class of respiration sensors utilizes a thermistor anemometer placed near the nose and/or mouth of the patient. U.S. Pat. No. 3,368,212 to Klyce (1968) showed that breath flow could be sensed by measuring the temperature of a thermistor placed in the respiratory airstream of a patient. Expired air from the patient's lungs, with it's elevated temperature, causes the thermistor temperature to rise. Inspired air causes the thermistor temperature to fall back toward ambient air temperature. Monitoring the rise and fall of the thermistor temperature gives a measure of the patient's respiration. Several patents have improved on this basic concept by improving placement of the thermistor and the use of other types of temperature sensors. U.S. Pat. No. 3,884,219 to Richardson et al (1975), U.S. Pat. No. 4,777,963 to McKenna (1988), and U.S. Pat. No. 5,190,048 to Wilkinson (1993) are examples of thermistor based breath sensors. U.S. Pat. No. 5,069,222 to McDonald (1991) is an example of a thermocouple based breath sensor. This class of breathing sensor is particularly advantageous in that it measures the actual exchange of air or respiratory flow. Other methods of detecting the presence of breathing such as chest wall movement or electrical chest wall impedance only infer the movement of respiratory air indirectly.
The large magnetic and RF electric fields present in an MRI system, however, make these breath sensing methods unusable due to image distortion caused by induced magnetic fields of the temperature sensor and associated cabling, heating caused by induced currents in the temperature sensor and associated cabling, and noise induced in the temperature sensor and associated cabling by changing electromagnetic fields. Indeed, any conductive or magnetic part placed in the bore of an MRI system is problematic because of the risk for patient burns, and the potential for image distortion compromising diagnostic quality.
A type of temperature sensor which is potentially unaffected by large magnetic and RF electric fields is a fiberoptic temperature sensor. Such sensors generally comprise an optical exciter and receiver system, a sensor tip which responds optically in some way when its temperature changes, and an optical fiber or fibers which couple the sensor tip to the optical exciter and receiver system. There are many examples of fiberoptic temperature sensors in the prior art. U.S. Pat. No. 4,223,226 to Quick et al (1980) showed that temperature at the end of an optical fiber could be determined by measuring changes in the emissions of a fluorescent phosphor placed at the tip of the optical fiber. U.S. Pat. No. 4,652,143 to Wickersheim et al (1987) demonstrated a fluorescent phosphor based fiberoptic sensor system and incorporated it into a commercial instrument for Luxtron Inc. Zhang et al (1993) demonstrated a fluorescent sensor based on chromium doped lithium strontium aluminum fluoride, which was particularly sensitive at typical physiological temperatures. Leilabady et al (1987) showed that a singlemode optical fiber Fabry-Perot interferometer could be used to measure temperature.
Present commercial implementations of fiberoptic temperature sensors are not suited for breath sensing because: a) they often lack the required response time to follow breathing rates, especially those of hyperventilating patients, children, and neonates, b) they often lack the required sensitivity to resolve the small thermal variations in a patient's respiratory airstream, c) they are designed for very accurate measurement of temperature over a broad range and are thereby excessively costly, d) the sensor tips themselves are relatively expensive and thereby cannot be treated as a disposable item, which is often necessary in the medical environment to minimize the spread contagious disease, e) some devices employ conductive and/or magnetic materials in the sensor tip, mitigating their use in an MRI environment, and f) they lack a sufficiently long optical fiber to span the distance between the operator console and the MRI magnet bore. Thus, none of these implementations anticipate their use for breath monitoring or breath monitoring during MRI imaging.